White polyolefin compositions having reduced die drool

ABSTRACT

Filled polyolefin compositions suitable for use in extrusion/coextrusion processes which reduce die drool are provided. More particularly, the compositions are white or opaque compositions comprising a polyolefin, whitening or opacifying inorganic filler and a mercaptobenzimidazole compound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to white polyolefin compositions useful in extrusion processes which exhibit reduced die lip build-up during extrusion. More particularly, the compositions contain a polyolefin, a whitening or opacifying filler and a die drool-reducing amount of mercaptobenzimidazole compound

2. Brief Description of the Prior Art

Inorganic fillers are commonly incorporated in polyolefin resins for a variety of purposes. For example, inorganic fillers may be employed to reduce cost, impart flame retardancy, improve modulus or toughness, impart opacity, impart printability/writeability, impart ultraviolet (UV) resistance and the like. The amount and type of polyolefin used and inorganic filler incorporated can vary widely depending on the intended end-use application.

Whitening or opacifying inorganic fillers, such as titanium dioxide and calcium carbonate, are widely used in the production of white or opaque polyolefin packaging films. Films of this type may be used individually or incorporated as one of the layers in a coextruded multi-layer film construction. White films are commonly employed where a white background for printing is desired and to protect packaged foods which are sensitive to light exposure.

The problems associated with the high-speed extrusion of filled polyolefins and subsequent forming operations are well documented. For example, U.S. Pat. No. 5,277,970 describes the tendency of opaque, vacuole-containing films to develop undesirable, stripe-like markings of varying intensity in the longitudinal direction when processed on high-speed automatic packaging machines and discloses multi-layered white film resistant to the development of such marking.

The presence of inorganic fillers is also known to contribute to undesirable die lip build-up during extrusion/coextrusion of the film. Dip lip build-up, also referred to as die drool, die drip or plate out, is the undesirable accumulation of polymer on the open face of the die. Die lip build-up can change the geometry of the die exit and cause surface roughness. Also, the built-up material can periodically break free and cause localized surface defects. If die drool is excessive it may require periodic shut down to clean dies and/or it may require operating at reduced line speeds. A discussion of the problems associated with die lip build-up and the various factors which contribute to this phenomenon and a review of common methods suggested to minimize or eliminate die lip build-up problems is provided in a paper presented by J D. Gander and A. J. Giacomin, SPE ANTEC, Indianapolis (May 1996), pages 1113-1117.

One approach to reducing die drool is the use of additives that function as processing aids/lubricants for the polyolefin. Fluoropolymer additives have been widely used for this purpose. There is a continuing need to provide new and improved white, filled polyolefin compositions which exhibit reduced die lip build-up upon extrusion. It would be particularly desirable if these compositions utilized readily available and low cost additives.

Benzimidazole compounds have been used as stabilizers for polyolefins to prevent degradation caused by oxidative, thermal and ultraviolet exposure. For example, U.S. Pat. No. 3,218,276 discloses polypropylene fiber-forming compositions protected against thermal and ultraviolet degradation containing 0.2 to 2 percent C₃₋₂₀ alkyl-substituted benzimidazole. U.S. Pat. No. 2,997,456 teaches the use of metallic mercaptobenzimidazole compounds as stabilizers for polymers of 1-olefins, primarily polypropylene, to protect against molecular degradation under conditions of elevated temperature and/or mechanical working. Zinc mercaptobenzimidaole is specifically mentioned.

Combinations of hindered phenols with various zinc salts of mercapto compounds to stabilize crosslinkable polyolefin insulation compositions is taught in U.S. Pat. Nos. 4,260,661; 4,693,9837; 4,797,323 and 4,824,883. For example, combinations of IRGANOX 1010 with the zinc salts of 2-mercaptobenzimidazole and 2-mercaptotoylimidazole are disclosed.

Other references which disclose benzimidazole stabilizers for crosslinkable elastomeric polymeric materials include U.S. Pat. Nos. 4,459,380; 4,808,643 and 5,196,462.

U.S. Pat. No. 6,197,852 discloses the use of mercaptobenzimidazole compounds for carbon black-filled concentrates having improved resistance to thermo-oxidative degradation and improved dispersion of carbon black.

SUMMARY

It is an object of the present invention to provide polyolefin compositions suitable for use in extrusion processes which exhibit reduced die drool. It is a further object to provide white or opaque polyolefin extrusion compositions having reduced die lip build-up.

The improved compositions of the invention, which can include both concentrates and let-down resins produced therefrom, comprise 40 to 98.5 weight percent, based on the weight of the composition, polyolefin resin selected from the group consisting of ethylene homopolymers, ethylene copolymers, propylene homopolymers and propylene copolymers, said polyolefin having a melt index from 0.5 to 50 g/10 min; 1.5 to 60 weight percent, based on the weight of the composition, whitening/opacifying inorganic filler; and 0.01 to 1 weight percent, based on the weight of the composition, mercaptobenzimidazole compound of the formula

where A is hydrogen or zinc, R is a C₁₋₄ alkyl group, n is 0 to 4 and x is 1 or 2.

The mercaptobenzimidazole compound is preferably 2-mercaptotolylimidazole, 2-mercaptobenzimidazole, zinc-2-mercaptobenzimidazole or zinc 2-mercaptotolyl-imidazole. Useful whitening/opacifying inorganic fillers include titanium dioxide, calcium carbonate, barium sulfate, silica, silica dioxide, talc, mica and kaolin.

Compositions containing titanium, dioxide and calcium carbonate fillers, especially those wherein the average particle size of the filler is in the range 0.1 to 10 microns, are particularly useful.

Especially useful extrusion compositions contain 45 to 97.5 weight percent low density polyethylene resin or linear low density polyethylene resin, 2.5 to 55 weight percent filler and 0.05 to 0.5 weight percent mercaptobenzimidazole compound. It is even more advantageous if these compositions contain 0.1 to 1 weight percent processing aid and/or 0.1 to 1 weight percent hindered phenol stabilizer.

DETAILED DESCRIPTION

The present invention relates to white polyolefin compositions having reduced die drool. As used herein the term white polyolefin compositions refers to compositions which are white or opaque. The degree of whiteness/opacity can vary depending on the type and amount of the inorganic filler incorporated in the polyolefin.

The compositions in their broadest terms are comprised of a polyolefin capable of being extruded, an inorganic filler capable of imparting the desired degree of whiteness/opacity and a die drool-reducing amount of mercaptobenzimidazole compound. Optionally, one or more additional additives commonly used in extrusion compositions, such as stabilizers and processing aids, may be included in the composition.

Conventional polyolefin resins suitable for use in extrusion processes are employed for the invention. These include ethylene homopolymers and copolymers, propylene homopolymers and copolymers and mixtures thereof having melt indexes (MIs) in the range from about 0.5 to 50 g/10 min determined in accordance with ASTM D 1238-01, condition 190/2.16. It is particularly advantageous to use polyolefins having MIs from about 2 to 40 g/10 min.

Useful ethylene homopolymers and copolymers within the specified MI range are available from commercial sources and include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE) and high density polyethylene (HDPE). Copolymers contain minor amounts of C₃₋₈ α-olefin comonomers such as propylene, butene-1, hexene-1 and octene-1; however, copolymers of ethylene with comonomers having functional groups such as vinyl carboxylates, alkyl acrylates and alkyl methacrylates may also be used. Ethylene-vinyl acetate and ethylene-n-butyl acrylate copolymers are highly useful copolymers as are copolymers of ethylene with butene-1 and hexene-1.

Densities of the ethylene homopolymers and copolymers can be as high as 0.965 g/cm³ but are most advantageously from 0.910 to 0.950 g/cm³. With highly filled compositions, such as when concentrates or masterbatches are prepared, LDPEs and LLDPEs having densities from 0.915 to 0.930 g/cm³ are highly useful. LDPEs having densities from 0.915 to 0.925 are particularly advantageous for formulating highly filled compositions in view of their ready processability and the ease of incorporating the high filler loadings. Densities recited herein are determined in accordance with ASTM D 1505.

Useful propylene polymers include polypropylene (PP), i.e., propylene homopolymer, random copolymers of propylene with ethylene or a C₄₋₆ α-olefin and impact propylene-ethylene copolymers which can include thermoplastic polyolefins (TPOs) and thermoplastic polyolefin elastomers (TPOEs). The impact copolymers can be either reactor-made intimate mixtures of polypropylene and ethylene-propylene copolymer or conventional blends obtained by physically mixing the polypropylene and ethylene-propylene copolymer components.

The polyolefin resin(s) will constitute 40 to 98.5 weight percent (wt. %) and, more preferably, from 45 to 97.5 wt. % of the total composition. The filler, which can be any inorganic filler capable of imparting the desired degree of whiteness or opacity to the finished product, comprises 1.5 to 60 wt. % and, more preferably, 2.5 to 55 wt. % of the total composition.

Inorganic fillers which can be used include any of the filler materials known to be useful for whitening/opacifying purposes. By way of illustration, these can include titanium dioxide, calcium carbonate, barium sulfate, silica, silica dioxide, talc, mica, kaolin and the like.

In one highly useful embodiment of the invention the inorganic filler is titanium dioxide (TiO₂) which can be of either the anatase or rutile form. Due to its superior opacity, the rutile form is preferably employed. The titanium dioxide may be surface treated, i.e., coated in accordance with known procedures. For example, the titanium dioxide particles may be coated with organic compounds or inorganic oxides where specific applications dictate. In another highly useful embodiment, the inorganic filler is calcium carbonate.

Average particle size of the inorganic filler will typically range from about 0.05 up to about 50 microns. While smaller particle size fillers provide a greater whitening/opacifying effect and better performance, they are typically harder to disperse. White compositions wherein the whitening/opacifying filler, particularly TiO₂ or calcium carbonate, has a particle size in the range 0.1 to 10 microns are particularly useful.

A mercaptobenzimidazole compound is included in the formulation with the polyolefin base resin and inorganic filler in order to achieve the reduced die lip buildup with the compositions of the invention. Useful mercaptobenzimidazole compounds for the invention correspond to the following formula

where A is hydrogen or zinc, R is a C₁₋₄ alkyl group, n is 0 to 4 and x is 1 or 2. Preferably, R is methyl and n is 0 or 1. Preferred mercaptobenzimidazole compounds include 2-mercaptotolylimidazole (MTI), 2-mercaptobenzimidazole (MBI), zinc-2-mercaptobenzimidazole (ZMBI) and zinc 2-mercaptotolylimidazole (ZMTI). Whereas mercaptobenzimidazoles of the above types have been used as stabilizers for crosslink able polymers and as an adjuvant to facilitate dispersion of carbon black in carbon black concentrates they have not been utilized in white/opaque formulations to reduce die drool.

The mercaptobenzimidazole compound is used in an amount from 0.01 to 1 wt. %, based on the total composition. In a highly useful embodiment compositions exhibiting significantly reduced die lip build-up contain from 0.05 to 0.5 wt. % of the mercaptobenzimidazole compound. The amount of mercaptobenzimidazole compound required to effectively reduce die drool will depend on the particular polyolefin and inorganic filler used and the relative percentages of each, the type and the amount of other resins/additives included in the formulation, the particular application involved and processing/operating conditions employed. As a general rule, however, all other things being equal, die lip build-up increases as the amount of filler is increased. Accordingly, it may be advantageous to use higher levels of the mercaptobenzimidazole compound and include other optional processing aids when formulating highly filled white compositions.

In one embodiment of the invention compositions useful for extrusion coating photographic papers will contain 85-90% LDPE, 10-15% TiO₂ and 0.05-0.5% mercaptobenzimidazole compound. In another useful embodiment, compositions useful as extrusion coatings for Kraft papers and the like contain 90-95% LDPE, 5-10% calcium carbonate and 0.05-0.5% mercaptobenzimidazole compound.

Conventional additives may be included in the composition in accordance with known formulation procedures. These additives can include antioxidants, light and heat stabilizers, processing aids, nucleating agents, flame retarding agents, metal deactivators and the like. Additives of this type are known in the art and are generally used at relatively low levels, i.e., not exceeding 2% on a combined basis. Since mercaptobenzimidazole compounds are known to impart antioxidative properties, incorporation of additional antioxidants and stabilizers may not be necessary except for the most rigorous applications and/or processing conditions.

In one highly useful embodiment of the invention 0.1 to 1 wt. % of a hindered phenol stabilizer is included in the formulation. Useful hindered phenols used will contain one or more substituted phenyl groups of the formula

where R is a C₁₋₄ alkyl group and, most preferably, a tertiary butyl group. Where more than one, 3,5-dialkyl-4-hydroxyphenyl group is present, they will be joined through a linking group and the resulting compounds will correspond to the formula

where n is an integer from 2 to 4 and L represents the linking group.

Representative linking groups can include:

It is especially advantageous when the above-identified linking moieties are substituted with 3,5-di-t-butyl-4-hydroxyphenyl groups. Representative hindered phenol compounds of the above types include:

-   4,4′-methylenebis(2,6-di-t-butylphenol); -   tetrakis[methylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane; -   1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene; -   1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-s-triazine 2,4,6     (1H,3H,5H)trione; -   N,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propanyl]-hydrazine; and -   octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate.

All of the foregoing materials are commercially available. Tetrakis[methylene (3,5-di-t-butyl-4-hydroxyhydrocinamate)]methane and octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate are particularly useful hindered phenols which can be used in conjunction with the mercaptobenzimidazole compound for the compositions of the invention.

In another highly useful embodiment, 0.1 to 1 wt. % of a processing aid is included in the formulation. Conventional processing aids such as fatty acids, fatty acid metal soaps, fatty acid amides, long chain aliphatic amides, fatty acid esters, silicone oils, natural and synthetic hydrocarbon waxes, low molecular weight polyethylene waxes, low molecular weight aliphatic resins and combinations thereof can be used for this purpose. Compositions wherein a fatty acid metal soap, such as calcium stearate or aluminum stearate, is employed in combination with the mercaptobenzimidazole compound are particularly advantageous.

To prepare the compositions, all of the ingredients can be dry blended and used as such or, as is more typically the case, melt blended prior to use. This latter procedure insures intimate mixing of the filler, mercaptobenzimidazole compound and any optional additives prior to extrusion since many extrusion operations do not utilize extruders having sufficient mixing zones to insure homogeneity of the extrudate. Mixers capable of imparting high shear, such as Banbury mixers, twin screw extruders or the like, can be used to accomplish the melt blending. Another useful approach in preparing the compositions is to prepare a concentrate or masterbatch of the base resin or another readily processible polyolefin resin which contains the inorganic filler, mercaptobenzimidazole compound and any optional ingredients and then to “let down” the concentrate in the base resin during the melt blending operation to produce the final formulation.

The compositions of the invention can be utilized in any operation where a white-filled polyolefin is extruded through a single die opening, such as a slit or circular die, or multiple die opening. While they are particularly effective in reducing die lip build-up in film extrusion and coextrusion processes they can also be advantageously utilized for extrusion coating, wire and cable coating, blow molding, film blowing, fiber spinning and the like. Reduced die drool is also observed when a concentrate is processed and extruded, such as when the concentrate is pelletized using a strand cutter.

The following examples illustrate the invention more fully. Unless otherwise indicated, all parts and percentages reported in the examples are on a weight basis.

All compositions were prepared by first preparing a concentrate. Concentrates were prepared using a Farrel OOC Banbury mixer having a capacity of 2400 cc. All of the ingredients were dry-blended and the preheated (35° C.) chamber of the Banbury filled with the mixture. A pressure of 40 psi was then applied with mixing (125 rpm). When flux was achieved, i.e., the temperature of the mixture in the chamber reached approximately 132° C. (usually about 40-50 seconds), the ram was raised for 15 seconds and any material collected in the throat of the mixer scraped into the mixing chamber. Pressure was reapplied and mixing continued for at least 3 minutes or until the temperature reached 171° C. The melt was then pelletized at 182° C. using a 2.25 inch single screw extruder (L/D=23; 10 rpm) connected to an underwater pelletizer. The concentrates were then let-down into the appropriate amount of the polyolefin resin in order to obtain the extrusion composition.

EXAMPLE 1

A white composition suitable for extrusion coating and coextrusion was prepared in accordance with the invention. The composition was comprised as follows: 50% LDPE (MI 5.6 g/10 min; density, 0.923 g/cm³) 25% LDPE (MI 9 g/10 min; density, 0.923 g/cm³) 24.7%   TiO₂ (Rutile; average particle size 0.2 microns) 0.2%  Aluminum Stearate (AlSt) 0.1%  Zinc 2-mercaptotolylimidazole (ZMTI)

The composition was prepared by letting down one part of a concentrate prepared as described above and containing 50% TiO₂, 49.4% of the 9 MI LDPE, 0.4% AliSt and 0.2% ZMTI into one part of the 5.6 MI LDPE.

To evaluate die lip build-up the composition was then extruded at 235° C. using a Haake twin screw equipped with a 1½ inch slit die with 40 mil opening. The extruder was operated at 150 rpm. As the extrusion proceeded, the die lip was examined after one hour and at 30 minute intervals thereafter and the width of the amount of drool accumulated on the lip of the die measured. The amount of accumulated die drool was then reported as a percentage of the overall die width. Die drool results obtained for the white composition prepared above were as follows: 60 minutes 5% build-up 90 minutes 15% build-up 120 minutes 30% build-up 150 minutes 40% build-up 180 minutes 45% build-up

COMPARATIVE EXAMPLE 2

To demonstrate the significance of the above results a comparative formulation was prepared and evaluated. The comparative formulation was identical in all respects to that of Example 1 except that the ZMTI was omitted and replaced with 0.1% of a hindered phenol antioxidant (Irganox 1010)—a conventional stabilizer additive commonly used in filled polyolefin compositions All of the procedures used to prepare and evaluate the comparative composition were identical to that described for Example 1. Die lip build-up results obtained for the comparative composition were markedly inferior to those obtained with the composition of the invention as evidenced by the following test results. 60 minutes 40% build-up 90 minutes 80% build-up 120 minutes 90% build-up 150 minutes 95% build-up

EXAMPLES 3 AND 4 AND COMPARATIVE EXAMPLE 5

Following the procedure described for the preparation of white concentrates, the following compositions were prepared. Example 3 Example 4 Comparative Example 5 LDPE 50 50 50 TiO₂ 49.3 49.3 49.3 AlSt 0.3 0.3 0.3 ZMTI 0.2 — — MTI — 0.2 — Irganox 1010 — — 0.2

The LDPE used had an MI of 9 g/10 min and density of 0.923 g/cm³. Rather than letting the concentrates down into additional resin, the concentrates were evaluated directly for die lip build-up during extrusion. Extrusion of highly filled compositions is considered to be a more rigorous test and considerably shortens the time required for testing. Die lip build-up results obtained for the above white concentrates were as follows: Percentage Die Lip Build-up Extrusion Time Example 3 Example 4 Comparative Example 5 5 minutes 25 30 80 10 minutes 30 45 90 20 minutes 45 50 95 40 minutes 50 50 98

The marked reduction in die drool obtained with the white compositions of the invention is apparent from the above results.

EXAMPLE 6

When a mercaptobenzimidazole compound is employed in conjunction with a hindered phenol compound, comparable reductions in die drool are possible. For example, using 0.2% of a mixture comprised of equal parts of ZMTI and Irganox 1010 in a concentrate containing 50% LDPE, 49.3% TiO₂ and 0.3% AlSt gives results comparable to those of Examples 3 and 4. 

1. A polyolefin composition comprising: (a) 40 to 98.5 weight percent, based on the weight of the composition, polyolefin resin selected from the group consisting of ethylene homopolymers, ethylene copolymers, propylene homopolymers and propylene copolymers, said polyolefin having a melt index from 0.5 to 50 g/10 min; (b) 1.5 to 60 weight percent, based on the weight of the composition, whitening/opacifying inorganic filler; and (c) 0.01 to 1 weight percent, based on the weight of the composition, mercaptobenzimidazole compound of the formula

where A is hydrogen or zinc, R is a C₁₋₄ alkyl group, n is 0 to 4 and x is 1 or
 2. 2. The composition of claim 1 wherein (b) is selected from the group consisting of titanium dioxide, calcium carbonate, barium sulfate, silica, silica dioxide, talc, mica and kaolin.
 3. The composition of claim 1 wherein R is methyl and n is 0 or
 1. 4. The composition of claim 1 comprising 45 to 97.5 weight percent (a), 2.5 to 55 weight percent (b) and 0.05 to 0.5 weight percent (c).
 5. The composition of claim 4 wherein (b) is titanium dioxide and (c) is selected from the group consisting of 2-mercaptotolylimidazole, 2-mercaptobenzimidazole, zinc-2-mercaptobenzimidazole and zinc 2-mercaptotolylimidazole.
 6. The composition of claim 5 wherein (a) is low density polyethylene or linear low density polyethylene having densities from 0.915 to 0.930 g/cm³.
 7. The composition of claim 6 wherein the titanium dioxide has an average particle size from 0.1 to 10 microns.
 8. The composition of claim 4 wherein (b) is calcium carbonate and (c) is selected from the group consisting of 2-mercaptotolylimidazole, 2-mercaptobenzimidazole, zinc-2-mercaptobenzimidazole and zinc 2-mercaptotolylimidazole.
 9. The composition of claim 8 wherein (a) is low density polyethylene or linear low density polyethylene having densities from 0.915 to 0.930 g/cm³.
 10. The composition of claim 9 wherein the calcium carbonate has an average particle size from 0.1 to 10 microns.
 11. The composition of claim 1 which additionally contains 0.1 to 1 weight percent, based on the weight of the composition, processing aid.
 12. The composition of claim 11 wherein the processing aid is a fatty acid metal soap.
 13. The composition of claim 12 wherein the processing aid is aluminum stearate.
 14. The composition of claim 1 which additionally contains 0.1 to 1 weight percent, based on the weight of the composition, hindered phenol stabilizer.
 15. The composition of claim 14 wherein the hindered phenol stabilizer is selected from the groups consisting of 4,4′-methylenebis(2,6-di-t-butylphenol), tetrakis [methylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene, 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-s-triazine 2,4,6(1H,3H,5H)trione, N,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propanyl]-hydrazine and octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate. 